Airfoils in high-temperature environments, such as vanes and blades in the hottest rows of a gas turbine, require thermal protection and cooling. Thermal barrier coatings (TBCs) are used to reduce heat flux into the airfoil and allow hotter surface temperatures on the airfoil. Currently, TBCs can only be applied as thin layers, since thermal gradients cause differential expansion within the coating and between the coating and substrate, which weakens the coating and its adhesion to the substrate. However, a thin TBC means that a substantial amount of air or steam cooling of the component is needed to maintain temperature limits of the substrate.
One technology to increase TBC thickness while maintaining its integrity and adhesion is called a back-filled honeycomb. This is a metallic honeycomb attached to a metal substrate surface and filled with a ceramic thermal barrier material. Examples of this technology are found in U.S. Pat. Nos. 6,846,574; 6,641,907; 6,235,370; and 6,013,592. A prefabricated honeycomb structure can be welded to a substrate. Alternately, a honeycomb may be fabricated by depositing a metal-ceramic material in a mask on the substrate and heating it to produce cohesion and a solid-state diffusion bond with the substrate. Back-filled honeycomb technology provides a metal-to-ceramic friendly bond, and allows thicker thermal barrier coatings.
A prefabricated honeycomb structure is useful for relatively flat surfaces, but cannot be conveniently bonded to a curved surface, such as an airfoil surface. The honeycomb masking/deposition method as in U.S. Pat. No. 6,846,574 can be used on curved surfaces, but is difficult to apply on highly curved or sharp surfaces, such as the leading and trailing edges of an airfoil.